This may be a known snmpv3 issue. The 10.02.03.PTF_10.2.321 patch for Spectrum 10.2.3 addresses the following issues:
Snmp engine ID discovery
(DE334682, 00897842)
After reboot device can't be monitored with SNMPv3
(DE348697, 00912085)
Reset for SNMPv3 Authentication Fails.
(DE354700, 00969917)
Updating Profiles hangs.
(DE354982, 00996623)
Fails to process SNMP responses from different IP.
(DE355500, 00997837)
With changed source ip address, SS crashes on polling after snmpv3 reset
(DE357015)
Symptom: Due to changes in engineID on the device causing SNMPv3 authentication failures.
Resolution: Now Spectrum respects the engineID changes on the engineID.
(DE357367,01007244 )
Management Agent Lost Alarms in 10.2.3
(DE360439, 001007458)
SNMPv3: update engineID in Spectrum when it's changed on Device.
(DE360883)
Validate SNMPv3 profile fields during Profile creation.
(DE362691 )
Joe